Field of the Invention
The present invention relates to an improvement in a crankshaft bearing structure of an internal combustion engine.
Background Information
A crankshaft of an internal combustion engine is supported by plurality of main bearing units that are configured by a semicircular recess in the cylinder block and a bearing cap that is attached to the lower surface of the cylinder block. In general, the main bearing units are respectively disposed at both ends of the crankshaft and in positions between cylinders.
For example, in an in-line four-cylinder internal combustion engine, the crankshaft is generally supported by five main bearing units, No. 1 to No. 5; a pair of halved metal bearings is mounted to each main bearing unit, and the crankshaft is rotatably supported via the metal bearing.
In a crankshaft bearing structure having a plurality of such main bearing units, the bearing clearances in the main bearing units are set to be equal to each other.
However, in a situation where the crankshaft is being rotated at an extremely high speed, the rotating crankshaft is warped into an arcuate shape by an inertia load. Consequently, the bearing load in the No. 3 main bearing unit in the center becomes larger compared to the other main bearing units. In particular, in an in-line four-cylinder internal combustion engine, the piston of the No. 2 cylinder and the piston of the No. 3 cylinder move in phase, but when the pistons of two adjacent cylinders move in phase in this manner, the main journal between these two cylinders is displaced the most to the outer perimeter side; therefore, the bearing load in the No. 3 main bearing unit in the center becomes even greater. Therefore, there is a concern that bearing durability is threatened when, for example, the engine temperature is high and the engine is rotating at high speed.
When trying to avoid warping of the crankshaft described above, the number and the weight of the counterweights that are applied to the crankshaft is increased, which leads to an increase in the weight of the crankshaft.
Japanese Laid-Open Patent Application No. 2007-162637 (Patent Document 1) discloses increasing the rigidity of specific main bearing units such that the load distribution of each bearing part becomes equal, giving consideration to the fact that a large inertia load is input to specific main bearing units, in an in-line four-cylinder internal combustion engine employing a multi-link piston crank mechanism.
However, the Patent Document 1 does not disclose varying the bearing clearance in each main bearing unit.